A CRT is evacuated to a very low pressure and accordingly is subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the CRT. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply a compressive force to the sidewall of a faceplate panel of the CRT to redistribute some of the forces. The redistribution of the forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the panel. An implosion protection band is also beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension and the band causes compression in panel areas which otherwise would be in tension. The implosion protection band also provides a convenient structure on which to attach a degaussing coil that is used to remagnetize internal ferromagnetic components of the CRT.
It is known that the earth's magnetic field affects the paths of the three electron beams emitted by a CRT electron gun and may cause what is called "misregister" in a color CRT. A shadow mask having a multiplicity of apertures therethrough is located in proximity to a tricolor screen of the tube to assure that each of the three electron beams impacts the phosphor element of the proper light emitting color. Thus, for example, the electron beam which is modulated with red information impacts the phosphor element which emits red light. Because the electrons of the beams are charged particles, the earth's magnetic field has an influence on their trajectories which can cause the electrons to impact a phosphor of the improper color, causing misregister, thereby degrading the quality of the image display. For this reason, a magnetic shield, preferably an internal magnetic shield, or IMS, is used within the CRT to act in conjunction with the shadow mask and its frame to shield a substantial portion of the electron beams trajectories from the influence of the earth's magnetic field.
The shielding acts in the following way: the external magnetic field realigns the Weiss domains in the ferromagnetic material of the shadow mask, mask frame and IMS, creating an induced magnetic field that tends to oppose the action of the external field of the earth which has given rise to it. If the material of the ferromagnetic components, i.e., of the shadow mask, mask frame and the IMS, has high magnetic permeability, the induced field at least partially opposes the external field. Hence, the deleterious effect of the earth's magnetic field is reduced.
For more efficient compensation, the ferromagnetic material of the components has to be de-magnetized with a magnetic field that is provided by the degaussing coil, fixed by appropriate means to the rear of the tube and overlying the ferromagnetic components. One such arrangement is shown in FIGS. 1 and 2, in which a CRT 1 comprises a glass envelope having a faceplate panel 2 and a funnel 3 which includes a neck 4. The sidewall of the panel 2 is attached to the funnel 3 by a frit seal 5. An implosion protection band 6 surrounds the skirt of the panel 2. The band 6 has four mounting lugs 7 located at the corners of the tube to provide a means to affix the tube within a receiver (not shown). A deflection yoke 8 is attached to the funnel 3 of the tube to deflect the electron beams from an electron gun (not shown) located in the neck 4. A degaussing coil 9 having two loops 10 and 11 is fixed by appropriate means to the external surface of the tube 1. In FIGS. 1 and 2, the coil 9 is looped over the mounting lugs 7 and secured by means of a pair of holding straps 12. Alternatively, as shown in FIGS. 3-4, the coil 9 may be secured to the band 6 by means of the straps 12' or by clips 13 attached to retainers 14 adjacent to openings 15 formed in the band. The openings 15 are formed by either stamping or piercing, as is known in the art. The retainers 14 are then bent out of the plane of the band. The configuration of the coil 9 and its method of attachment to the tube depends on the tube parameters, and is adjusted to minimize misregister. As shown in the FIG. 1, the coil 9 overlies at least a portion of the implosion protection band 6.
one drawback of forming the openings 15 in the band 6 for attaching either the straps 12' or the clips 13 is that the stamping or piercing operation which forms the openings frequently causes a burr, or sharp projection, to be formed on the band 6 adjacent to the openings. Burrs are undesirable because they can either cut the insulation of the degaussing coil or injure anyone handling the tube, or if directed inwardly, toward the glass envelope of the CRT, may scratch the glass and weaken it so that it is more prone to implode if bumped or otherwise impacted. Removing the burr from the band 6 either by hand, or mechanical deburring, before installation of the band on the tube, adds additional manufacturing steps and cost to the tube manufacturing operation. Because it is nearly impossible to form the openings 15 without any burr, it is desirable to negate the effect of the burr without increasing tube cost.